Hologram reproduction system using an optical grating

ABSTRACT

A holographic image reconstruction system in which holographic light amplitude and phase information are resolved spearately for reconstruction of the original hologram. The amplitude component is separately detected and reproduced on a cathode ray tube or other electronically addressable optical display. The image of the amplitude component is projected through an external optical grating onto a film for recording or onto a real time imgaing system. Phase information from the holographic data is separately detected and superimposed on the amplitude data by varying the phase of the shadow image of the optical grating on the amplitude image. Phase moduflation of the image of the grating is accomplished by modulating the aperture or angle of the lens which images the amplitude pattern through the grating onto the film, or by modulating the grating itself.

Waited States Macovski atent 91 11 3,733,512 1 May 15, 1973 [54]HOLOGRAM REPRODUCTION 21 Appl. No.: 164,142

Related [1.8. Application Data [62] Division of Ser. No. 889,392, Dec.31, 1969, Pat. No.

U N ITED STATES PATENTS Kozma ..178/6.5

OTHER PUBLICATIONS A. Macauski, Ph. D. Dissertation, 1968, StanfordUniversity Primary Examiner-Samuel Feinberg Assistant ExaminerN.Moskowitz Attorney-Townsend & Townsend [57] ABSTRACT A holographic imagereconstruction system in which holographic light amplitude and phaseinformation are resolved spearately for reconstruction of the originalhologram. The amplitude component is separately detected and reproducedon a cathode ray tube or other electronically addressable opticaldisplay. The image of the amplitude component is projected through anexternal optical grating onto a film for recording or onto a real timeimgaing system. Phase information from the holographic data isseparately detected and superimposed on the amplitude data by varyingthe phase of the shadow image of the optical grating on the amplitudeimage. Phase moduflation of the image of the grating is accomplished bymodulating the aperture or angle of the lens which images the amplitudepattern through the grating onto the film, or by modulating the gratingitself.

4 Claims, 9 Drawing Figures PATENTEDHAY] 5 I375 SHEET 2 [IF 2 LIGHTSOURCE i l DRIVER ENVELOPE PHASE 2O DETECTOR k DETECTOR FIG 8 FIG .7

HOLOGRAM REPRODUCTION SYSTEM USING AN OPTICAL GRATING This is a divisionof application, Ser. No. 889,392, filed Dec. 31, 1969, now Pat. No.3,641,264.

This invention relates to a new and improved hologram reproductionsystem for reconstructing a holo graphic image from information aboutthe amplitude and phase of diffracted light rays obtained from ahologram or generated by a computer or data processing system.

In conventional holography, coherent light diffracted by an object orreflected from an object interferes with a reference beam of coherentlight to produce a complex light interference pattern known as ahologram which can be recorded on photographic film. The hologram,although not visually recognizable as a reproduction of the object,contains all the optical information necessary for reconstructing orreconstituting an accurate three-dimensional image or representation ofthe original object. This can be accomplished by exciting the hologramwith coherent light.

The hologram interference pattern contains informallUll about theamplitude and phase of the diffracted light rays, Hupcrittiposcd on itspatial frequency carrier whose spatial frequency is a function of theangle between the axes of the diffracted object beam and the referencebeam of coherent light. Thus, the intensity signal, i (x,y) across thediffraction pattern is a function of the following general form where Kis a constant, a(x,y) is the amplitude component containing amplitudeinformation about the diffracted light, l9(x,y) is the phase componentcontaining phase information about the object and reference light, and wis the spatial frequency of the carrier:

Conventional holography has the disadvantage that the amplitude andphase information is generally superimposed on a high spatial frequencycarrier so that special high resolution films must be used to record thehologram pattern. Such high resolution films are insensitive and requirelong expose times.

The spatial frequency carrier for the amplitude and phase informationcan be reduced by reducing the angle between the diffracted object lightbeam and the reference light beam to small angles approaching zero. Thissystem of holography, however, known as the Gabor on-axis system, hasthe disadvantage that at low spatial frequencies, separation of theamplitude and phase information about the diffracted object andreference light beams from undesirable optical information is poor.Furthermore, when the hologram is reproduced on a cathode ray tube, theresolution of the cathode ray tube limits the frequency of the spatialcarrier.

In US. Pat. application Ser. No. 781,842 filed on Dec. 6, 1968, entitledSCANNED HOLOGRAPHY SYSTEMS USING TEMPORAL MODULATION, the presentinventor sets forth an efficient holography system for superimposingholographic amplitude and phase information on a temporal frequencycarrier of the following general form in which the symbols correspond toequation l) except that the frequency w is a temporal frequency.

K K y) cos i ,y)

Briefly, according to the disclosure in that patent appli cation, aninterference pattern is established between light scattered from anobject and a reference light beam. One of the beams, however, is phasemodulated to cyclically offset in time, by displacing in phase, therelationship between the reference and object beams. The imageinformation is thereby superimposed on a temporal frequency carrier. Theresulting interference pattern is scanned to generate electrical signalsmodulated on a carrier, including the desired and undesired lightinformation which may thereafter be separated by electronic filtering.As a result, an on-axis system can be utilized while still permittingseparation of the desired amplitude and phase information from undesiredinformation. For hologram reproduction on a cathode ray tube, however,this system makes inefficient use of the CRT because the low frequencycarrier limits the resolution obtainable by the CRT.

It is, therefore, an object of the present invention to provide it newand improved holographic image reconstruction system which permitsseparation of optical image data from undiffracted optical components bythe use of a high frequency spatial carrier.

Another object of the invention is to provide a holographic reproductionsystem particularly suitable for reproduction by a cathode ray tube andwhich takes optimum advantage of the resolution capabilities of thecathode ray tube.

A further object of the invention is to provide a holographic imagereconstruction system in which amplitude and phase information areresolved and reconstructed separately. The amplitude component alone canthen be reproduced on a cathode ray tube or other electronicallyaddressable optical display so that the spatial frequency carrier doesnot have to be resolved by the optical display.

In order to accomplish these results, the present invention generallycontemplates a holographic image reconstruction system in whichholographic light amplitude and phase information in electrical analogsignal form are separately detected for use in reconstructing a hologramfrom the amplitude and phase data. An electrical signal containing thedesired holographic light amplitude and phase information in analog formcan be generated for example by an image dissector which temporallyscans a generated hologram interference pattern in the manner set forthin patent application Ser. No. 781,842 referred to above. Electricalanalog signals containing holographic light amplitude and phaseinformation can also be generated by computer and data processingequipment as is well known in the art.

According to the invention, the amplitude is envelope detected and theabsolute value of the amplitude signal applied to a cathode ray tube orother electronically addressable optical display. The cathode ray tubeor other display constitutes an incoherent light system in which theamplitude component alone is reproduced. A feature and advantage of theinvention is that the cathode ray tube or other display does not have toresolve the the spatial frequency carrier upon which the amplitude andphase data are ultimately superimposed. It only has to resolve therelatively low frequency signals representing the amplitude components.

The invention also contemplates the provision of an external opticalgrating in the form of, for example, a binary line grating or alenticular lens structure through which the amplitude image reproducedby the optical display is projected, for example, onto a film. Thus, theoptical grating which supplies the necessary spatial frequency carriercan be positioned adjacent a film plane, and the amplitude patternimaged through the external grating onto the film to form atransparency. Alternatively, it is a feature of the invention that theamplitude image can be projected through the grating onto a real timeimaging system for application in threedimensional television.

According to another aspect of the invention, the phase image data isalso separately detected and the phase signal applied for varying thephase of the shadow image of the grating on the film or real timeimaging system. For example, the image of the external grating on thefilm or imaging system can be phase modulated by modulating the apertureor angle of the lens which images the amplitude pattern through thegrating. Thus, the phase signal can be used, for example, to drive amotor which appropriately positions or modulates the aperture, or tocontrol light valves at the aperture. The result of the summation ofamplitude and phase information is a reconstruction of the hologram onfilm or instantaneously on a real time imaging system from which athree-dimensional reproduction of the original object can be obtained byexciting the hologram with coherent light.

The invention contemplates introduction of the phase information intothe reproduced amplitude image according to a variety of techniques.

In one form of the invention, the aperture of the image forming lens ismodulated by a sonic delay line placed in the exit pupil of the lenswith crossed polarizers on either side of the delay line. The sonicdelay line consists of a layer of material such as fused quartz or waterat one end of which a sonic pulse is generated in response to the phasecomponent extracted from the electrical signal. As the sonic pulsetravels along the material layer, the polarization changes in the regionof the pulse as a result of stress birefringence thereby providing aneffective transparent aperture which travels along the layer of thesonic delay line. The signal applied to the cathode ray tube in thatevent is not the amplitude envelope but rather the half-wave rectifiedpulses representing the amplitude data and a fast response CRT or otheroptical display must be used. The pulses applied to the sonic delay lineare synchronized with the rectified pulses representing amplitude datawhich are applied to the CRT or optical display. The pulses applied tothe sonic slit generator are at a constant frequency, while the phase ofthe data pulses applied to the optical display vary. The phaseinformation is, therefore, automatically superimposed on the amplitudepattern projected through the optical grating according to the angledetermined by the location of the transparent slit along the sonic delayline relative to the amplitude pattern at a particular time. This changein angle changes the phase of the grating shadow on the sequentialamplitude images.

In another form of the invention, the phase information and spatialfrequency carrier are superimposed on the amplitude information suppliedto a cathode ray tube within the cathode ray tube itself. According tothis embodiment of the invention phase deflection plates, and magneticfocusing elements are provided within the cathode ray tube in additionto scanning deflection elements, for controlling the electron beam. Thephase signal is applied to the phase deflection plates at the same timethat the amplitude signal is applied to the electron beam generator ofthe cathode ray tube so that both the amplitude and phase informationare superimposed on the electron beam. A wire grid is placed close tothe phosphor screen for superimposing a spatial frequency carrier on theelectron pattern impinging on the phosphor screen and the phase of thesuperimposed spatial carrier is caried by the phase signal applied tothe phase deflection plates. A feature and advantage of this embodimentof the invention is that the CRT does not have to resolve the spatialfrequency carrier as it is superimposed by shadowing.

According to another embodiment of the invention, an image isreconstructed from a hologram intensity function in electrical analogsignal form by synchronously detecting and separating the real andimaginary parts of the intensity function. The real and imaginary partsconsist of cosine and sine waveforms with both positive and negativevalues. Each of the real and imaginary parts is therefore separated, as,for example, by clipping, into positive and negative components and theabsolute value of each of the components is formed. The four componentsare thereafter sequentially applied to a cathode ray tube or otherelectronically addressable optical display to form sequential patternscorresponding to the component signals. Each of the four patterns ISimaged through an optical grating as heretofore described so that theshadow image of the optical grating is superimposed on the patternv Thefour superimposed patterns are summed, for example, on photographic filmor a real time imaging system to form the final image. As each of thefour patterns is imaged and superimposed on the optical grating, theposition of the aperture or exit pupil of the lens imaging system ischanged thereby to change the angle of projection according to thecomponent being reproduced on the cathode ray tube. Alternatively, theposition of the grating is varied to thereby vary the phase of thegrating image in accordance with the particular component patternreproduced by the cathode ray tube.

The invention also contemplates real time reproduction of athree-dimensional image from the reconstructed hologram for applicationin, for example, three-dimensional television. In this respect, theinvention contemplates imaging the amplitude and phase information on acontrolled transparency screen such as an Eidophor, a scannedelectro-optic crystal, or a photochromic plate. The changing real timeholographic sequence on the controlled transparency screen is excitedwith coherent light for continuous observation of a three-dimensionalrepresentation of the original obect.

In the Drawings:

FIG. I is a diagrammatic view of an optical train for constructing animage from holograph light amplitude and phase information according tothe present invention.

FIG. 2 is a diagrammatic view of another hologram reproduction systemfor reconstructing an image from the rectified hologram intensityfunction.

FIG. 2a is a graph showing the amplitude signal applied to the grid ofthe cathode ray tube in FIG. 2.

FIG. 3 is a diagrammatic view of a hologram reproduction system forconstructing an image from the real and imaginary components of ahologram intensity function.

FIG. 4 is a diagrammatic view of another hologram reproduction systemfor constructing an image from holographic light amplitude and phaseinformation.

FIG. 5 is a fragmentary, diagrammatic cross-sectional view of a cathoderay tube modified for constructing an image from holographic lightamplitude and phase information according to the present invention.

FIG. 6 is a diagrammatic perspective view of a conventional arrangementfor producing a threedimensional image from a hologram transparency.

FIG. 7 is a diagrammatic view of an arrangement for real timereconstruction and display of threedimensional holography.

FIG. 8 is a diagrammatic view of a system for holographic reconstructionutilizing a generalized scanned optical display in lieu of a cathode raytube.

FIG. 1 is a diagrammatic view of a generalized system for reproducingholographic image data in electrical analog signal form according to thepresent invention. As shown in that figure, an electrical analog signalrepresenting a holograph intensity function i(t) is applied to envelopedetector 11. The intensity function i(t) is of the form set forth inequations (1) or (2) above and the output of envelope detector 11corresponds to the absolute value of the amplitude component a(x,y).This magnitude signal derived by envelope detecting the intensityfunction is used to modulate the intensity of the electron beamgenerated by cathode ray tube 12 to thereby form on the face of the tubean amplitude pattern representative of the amplitude information aboutthe image being reconstructed. The amplitude pattern formed on the faceof the cathode ray tube 12 is imaged by lens sytem 13 through anaperture 14 formed in mask 15. The image so formed is projected througha line optical grating 16 onto a film plane 17 positioned adjacent butspaced from the optical grating 16. The image formed on film plane 17 istherefore the amplitude pattern formed by the cathode ray tube with theshadow image of line optical grating 16 superimposed on the amplitudepattern. The optical grating thereby provides the necessary spatialfrequency carrier.

At the same time, the intensity function signal i(t) is phase detectedby phase detector 20 consisting of a synchronous detector such as aproduct detector or mixer and filters for extracting the phasecomponent. The phase signal is used to drive the precision motor 21which operates a reciprocating element 22 connected to the mask 15, inresponse to the phase signal. The motor 21 is constructed so that theextent of displacement of the mask 15 and aperture 14 by reciprocatingelement 22 is proportional to the phase signal. As a result of thedisplacement of the mask 15 the aperture 14 changes position in responseto the phase signal changing the phase of the shadow image of opticalgrating 16 superimposed on film pattern 17. Alternatively, the motor 21could drive the optical grating 16. As a result, phase information aboutthe image being reconstructed is introduced by varying the phase of thespatial frequency carrier superimposed on the amplitude patterngenerated by cathode ray tube 12.

A further understanding of image reconstruction from holographic imagedata according to the present invention is obtained by referring to thefollowing generalized intensity function f(t) for a holograminterference pattern in which R is the amplitude of the reference light,U the amplitude of the object light, and w the frequency of either atemporal or spatial frequency carrier, the following equation showing atemporal frequency carrier:

The R term represents the intensity of undesirable background referencelight while the U term represents the intensity of undesired objectedlight. The cross terms, however, represent the desired interferencefringes produced by interference of the object and reference light andcontaining the desired conjugate image information (R* U representingthe real image term and U*R representing the virtual image term). In USPat. application Ser. No. 781,842, entitled SCANNED HOLOGRAPHY SYSTEMSUSING TEMPORAL MODULATION, referred to above, the conjugate image termsare modulated or offset on a temporal frequency carrier or frequency wand the entire hologram intensity function is represented in electricalanalog signal form by scanning the hologram with a scanned hologramrecorder. The desired components of the signal can then be separatedfrom the undesired components by appropriate electrical filtering. Afterfiltering, the R and U terms are eliminated leaving the filtered signalf (t) containing the conjugate image information which in turn containsboth the amplitude and phase information necessary for reconstructing animage of the original object.

The magnitude of the filtered signal mag f,(t) which represents theabsolute value of the amplitude information contained in the signal isderived by envelope detecting the filtered intensity function to givethe following signal wherein Re represents the real portion or cosineportion of the generalized intensity function and wherein 1m representsthe imaginary or sine component of the generalized intensity function:

The filtered signal f (t) can thus be represented by separable amplitudeand phase components:

The phase information signal is derived by synchronously detecting theintensity function f(t) using the sine and cosine signals, sine wt andcos wt, of the intensity function and deriving the arctangent of theirratio as set forth in the following equations:

Such synchronous detection is accomplished utilizing a product detectoror mixer with appropriate filtering. The ambiguity of the arctangentfunction can be 7 resolved by using the polarity of the imaginarypan,f(t) sin wt, which is positive for the phases to 180 and negativefor phases 180 to 360. Thus, a nonlinear transfer function approximatingthe arctangent can be used to find the desired phase.

Preferably, the phase signal 0 can be derived by comparing a referencephase at the frequency w with the varying signal cos( wt+0). In thiscase, the phase detector must operate over 360. This can be done withappropriate shaping networks by deriving a pulse waveform from one ofthe signals, for example, the reference signal, and by deriving asawtooth waveform from the other signal, namely, the signal of variablephase, cos(wt+0). The pulse is then used to sample the sawtoothamplitude. The amplitude of the sawtooth wave at the sample time islinearly proportional to the phase difference. Thus, the sampled outputwill be a voltage equal to k0, where k is a constant.

The magnitude signal, derived from the envelope detector, is applied toa cathode ray tube. The phase information is applied to continuouslyvary the position of the aperture of the imaging system which images theamplitude pattern through the optical grating onto an adjacent filmplane in accordance with the output of the phase signal. As shown inFIG. 1, a precision motor is used to vary the position of aperture 14.Alternatively, the aperture can be quantized into an array of smallelongated light modulators or light valves which are individually openedand controlled by the phase signal to change the angle of projection ofthe image formed by cathode ray tube 12.

The geometrical requirements for the arrangement of the elements in FIG.1 according to ray tracing techniques are set forth in the followingEquation (7). a is the distance between the plane of the aperture orexit pupil l4 and the film or image plane 17, while d is the distancebetween the line optical grating 16 and the film or image plane 17. S1is the distance between lines on the optical grating 16 while S2 is theslit translation along the plane necessary to obtain a phasedisplacement of 360 or one wavelength for the shadow image of grating 16on plane 17.

S (1 S d The aperture of the imaging system can also be modulated byusing continuously varying optical systems.

Such a system for continuously varying the aperture of the imagingsystem is illustrated in FIG. 4. As shown therein, a plate 30 comprisingan electro-optic valve for selectively polarizing light is positionedadjacent the face of cathode ray tube 31. Light from the amplitudepattern formed on the cathode ray tube 31 is selectively polarized bypolarizing valve 30 according to the phase signal applied to theelectro-optic valve driver 32. The polarized light is imaged by lenssystem 33 through a linear polarizer such as polaroid film comprisinglines of continuously varying polarization. Thus, linear absorber 34comprises a row of lines 35, each line having a slightly difierentdirection of polarization to provide continuously varying polarizationin a direction perpendicular to the lines so that the effective lineaperture or exit pupil for light imaged by lens 33 varies in response tothe phase signal applied to electro-optic polarizing plate 30. The imageis thereafter projected through the line optical grating 36 ontoadjacent film plate 37 as heretofore described.

Alternatively, the mask 34 could comprise a sheet of lines ofcontinuously varying color in a direction perpendicular to the lines,with plate 30 comprising a chromatic birefringent cell for varying thecolor transmission in accordance with the phase signal applied to driver32. The etfective position of the line aperture or exit pupil for lens33 would thereby be continuously varied in response to the phase signalapplied to driver 32.

Another system for introducing phase information into the reconstructedimage is shown in FIG. 2. According to that embodiment, the holographintensity function in the form of an electrical analog signal is appliedto the input line 40 to the electron beam generator of cathode ray tube41. Interposed in line 40 is a diode 42 for rectifying the input signalso that the signal input to the electron gun of cathode ray tube 41 isin the form of rectified pulses modulated by the amplitude envelope asillustrated in the graph of FIG. 2a. Thus, the signal consists of asequence of pulses having an average period (2w)/w, where w is thefrequency of the holographic image information carrier. The amplitude ofthe pulses is, of course, modulated according to the image amplitudeinformation, while the phase of the pulses varies according to thecorresponding image phase information. The resulting patterns formed bythe application of the signal shown in FIG. 2a to the electron gun ofcathode ray tube 41 are imaged by lens system 43 through a sonic delayline 44 interposed between a pair of crossed polarizers 45. The sonicdelay line 44 and crossed polarizers 45 function together to form adynamic aperture or exit pupil for the lens sytem 43. Sonic delay line44 consists of a layer of material such as fused quartz or water alongone end of which sonic pulses are generated by an elongated transducer46 in response to the square wave pulses of a pulse signal having aconstant phase and a constant period between the pulses of (2w)/w. Thepulse signal, represented at 47 in FIG. 2, is applied at line 48. As thegenerated sonic pulse travels along the material layer 44, thepolarization changes in the region of the pulse as the result of stressbirefringence, thereby providing an effective transparent slit or lineaperture which travels along the layer of the sonic delay linepermitting light to pass through the crossed polarizers. The pulsesapplied to the cathode ray tube are of variable phase so that thepatterns produced by the cathode ray tube are imaged by lens system 43through transparent slits or line apertures formed by the functionalcombination of the sonic delay line in different positions according tothe variation in phase of the pulses applied to the electron beamgenerator of cathode ray tube 41. The images of the amplitude patternsformed by lens system 43 are projected through the line optical grating50 onto an adjacent film plane 51 as heretofore described. Thus, thepulses applied to the sonic delay line are synchronized with thefrequency of the carrier of the holographic image data but the pulsesapplied to the sonic slit generator are at constant frequency and phase,while the phase of the data pulses applied to the cathode ray tube vary.The phase information is therefore automatically superimposed on theamplitude pattern projected through the optical grating because of thechange in angle depending upon the location of the transparent slitalong the sonic delay line 44 with reference to the pattern being formedon cathode ray tube 41.

The geometry of the elements of the system is arranged so that thedistance D2 between the film plane 51 and the plane of the effectiveexit pupil of lens system 43, the distance of travel of the generatedslit during one pulse period equal to vT where v is the velocity ofpropagation of the acoustic pulse in the sonic slit generator and T isthe period between pulses, the distance D1 between the film plane 51 andline optical grating 50 are all related according to the followingequation:

S1 d2 VTd In the embodiment of the present invention shown in FIG. 5,signals representative of the amplitude and phase extracted from theholograph image intensity function are applied separately forreconstruction of an image within the cathode ray tube itself. As shownin that Figure, the signal obtained by envelope detecting the holographintensity function signal and representing the amplitude imageinformation is applied to the electron gun 61 of cathode ray tube 60 formodulating the intensity of the generated electron beam. The cathode raytube is constructed so that in addition to the normal scanningdeflection yoke 63, there are also provided magnetic focusing elements62 and phase deflection plates 64. A phase signal, obtained bysynchronously detecting the holographic image intensity function signalas heretofore described is applied to the phase deflection plate 64 atthe same time that the amplitude signal is applied to the electron gun61. A wire grid 65 is placed close to but spaced from the phosphorscreen 66, or screen of controlled transparency material, so that thestructure of the grating 65 is superimposed by shadowing onto thepattern formed by the scanning electron beam. The grating 65 ispositioned substantially normal to the direction of the scan lines ofelectron beam 67 in order to avoid moire patterns. The phase deflectionplates 64 and magnetic focusing system 62 function together to vary thephase of the shadow image of grating 65 on the pattern formed by theelectron beam. While the electron beam 67 is intensity modulated withthe envelope information superimposing an amplitude pattern on thescanning electron beam the phase information signal is applied to theauxiliary phase deflection plate 64. These plates along with themagnetic lens system 62 determine the angle at which the beam strikesthe wire grid and thus the phase of the shadowed image of the grating onthe amplitude pattern formed on phosphor screen 66. A feature andadvantage of this system is that the electron beam does not resolve thegrating structure because the grating structure is superimposed on theamplitude pattern by shadowing of the electron beam. Thus, the beam canbe much larger than the period of the grating.

Another approach to reconstructing an image from a holograph imageintensity function is shown in the system of FIG. 3. According to thisaspect of the invention a holograph image intensity function f (t) ofthe form,

obtained by filtering as heretofore described is separated into real andimaginary components using synchronous detector. One synchronousdetector extracts the cosine or real component of the intensity functionas follows:

Ref (t) cos wt [U*R R*U"""] U*R R*U high frequency components which arefiltered out) second harmonic terms and higher order terms beingexcluded from the result by appropriate filtering. The secondsynchronous detector extracts the sine or imaginary component of theintensity function as follows,

IMf (t) sin wt U*Re' R*Ue =i [U*R R *U high frequency components whichare filtered out) second harmonic terms and higher order terms beingexcluded from the final results by appropriate filtering. Thus,according to a generalized form of this embodiment of the invention theholographic image intensity function is applied to a pair of synchronousdetectors 70, the output of which passes through a filter system 71including low pass filters, and the results are applied sequentially tothe electron gun of cathode ray tube 72 for modulating the intensity ofthe electron beam. The patterns produced by the cathode ray tube 72 areimaged by lens system 73 through line optical grating 74 having aspatial frequency w positioned immediately adjacent the film plane 75.First, with the grating in a fixed initial position the real componentsignal is applied to the cathode ray tube and is scanned utilizing a coswt function. The grating 74 is then moved horizontally a distance ofone-quarter of a wavelength, namely, M4 and the imaginary componentsignal is applied to the cathode tube and scanned utilizing a sin wtfunction. The resultant image intensity function superimposed on filmplane 75 is given by the following equations:

Thus, the terms have been separated on a spatial frequency carrier offrequency w,,, which the cathode ray tube did not have to resolve. Thecathode ray tube only had to resolve the low frequency signalsrepresenting the real and imaginary parts or components of the imageintensity function.

A disadvantage of this generalized system described above is that thereal and imaginary components of the image intensity function are ACwaveforms with both positive and negative values. To insure that theentire signal modulates the cathode ray tube a bias term must be addedto both the real and imaginary parts. This causes a spot to appear inthe center of the reconstructed image since a fixed carrier will begenerated. This bias problem can be overcome if the absolute value ofthe real and imaginary parts are applied to the cathode ray tube. Thepolarity of the grating must then be reversed each time the polarity ofthe real or imaginary part reverses.

A system for separately imaging both the positive and negative parts ofthe real and imaginary components of a holographic image intensityfunction is further shown with reference to FIG. 3. In the aperture orexit pupil of lens system 73 is placed a series of elongated lightvalves 76 in number equal to the four separate parts of the imageintensity function being reproduced. With only one of the light valves 1through 4 open, a relatively high F number or narrow angle opticalsystem is provided. With the narrow slits provided by the four lightvalves, the grating 74 is shadowed onto the film plane 75 in differentpositions depending upon which of the light valve slits is open. Thus,the positions of the open slit formed by the respective light valvesdetermines the phase of the grating shadow image on photograph film 5.In this method, the conventional line optical grating 74 is placed veryclose to but not against the film plane 75. In utilizing the system,slits l. and 3 can be used, for example, in imaging, respectively, thepositive and negative polarities of the cos wx component while the slitsprovided by light valves 2 and 4 can be used, respectively, for thepositive and negative polarities of the sin wx component. Negativecomponents of the sin and cos signals obtained from synchronousdetectors 70 are first passed through polarity reversal components 78for application to the cathode ray tube.

In order to obtain proper phase relationship between the shadow image ofgrating 74 and the four patterns produced by cathode ray tube 72 andimaged onto film plane 75 by means of the lens system 73 and lightvalves 76, the geometry of the system must be arranged accordingly.Where d is the distance from the exit pupil formed by light valves 1through 4 and film plane 75, a is the distance between optical grating74 and film plane 75, .S', is the distance between lines on the lineoptical grating 74 and S, is the distance over which the exit pupil slitposition changes a distance of one wavelength or 360 phase shift, then,the geometry of the elements must be arranged so that:

The grating structure 74 near the film plane 75 can alternatively beeither a density grating or binary grating or a lenticular lensstructure. The latter has greater optical efficiency and providesgreater depth of modulation of the resultant superimposed gratingbecause of the focusing action of the lenticular lens structure. Itdoes, however, have a slight defocusing action on the cathode ray tubeimage itself.

In operating the system of FIG. .3, the absolute value of one of thecomponents, for example, the real component, of the image intensityfunction is applied to the cathode ray tube. A clipped polarity" signalis derived from polarity signal generator using the polarity of the partof the real component being applied to the cathode ray tube and thisclipped polarity signal is utilized for turning on either light valve lor 3, depending upon the polarity of the part of the real componentbeing applied. A pattern is thereby formed on the cathode ray tubescreen by the scanning action of the electron beam. In the second scan,the absolute value of the imaginary component of the image intensityfunction is applied to the cathode ray tube with a clipped polaritysignal turning on either light valve 2 or 4., depending on the polarityof the part of the imaginary component being applied to the cathode raytube. Thus, one of the light valves 1 or 3 is open during scanning ofthe positive part of the real component while the other light valve isopen during scanning of the negative part of the real component.Similarly, one of the light valves 2 or 4 is open during scanning of thepositive part of the imaginary component of the image intensityfunction, while the other light valve is open during scanning of thenegative part of the imaginary component of the image intensityfunction. Each light valve provides an effective phase shift of thegrating shadow of M4 or 90 so that the positive and negative parts ofthe sin and cos components are superimposed in the proper phaserelationship. The respective scan for the real and imaginary componentscan be formatted either as complete alternate frame scans or can beformatted on a line-by-line basis with a line advance occurring onlyafter line scans for both the real and imaginary components have takenplace.

Thus, according to the system set forth above, the cathode ray tube onlyhas to resolve the low frequency signals representing the real andimaginary parts and does not have to resolve the spatial frequencycarrier superimposed on the reconstructed amplitude information byshadowing on film plane 75.

In each of the foregoing examples, the amplitude pattern imaged throughthe phase grating can be projected on a film plane for summing andstoring the holographic information on a film transparency. As shown inFIG. 6. a three-dimensional image of the original object is obtainedfrom such a transparency by passing a beam of coherent light generatedby laser 81 through the hologram transparency at an angle with respectto the perpendicular axis of the transparency. At the same time, thetransparency is viewed from observation point 82 substantially along theoptical axis of transparency 80 for viewing the three-dimensionalvirtual image 83 of the original object.

Rather than permanent storage of the reconstructed hologram on a filmtransparency, the hologram can be imaged on a screen of material ofcontrollable transpar ency such as an Eidophor, a scanned electro-opticcrystal plate. or a photochromic plate. Controllable transparencyscreens of this type are well-known in the art as set forth in thefollowing references which describe, respectively, the use of Eidophor,electro-optic and photochromic screens:

l. E. I. Sponable, EIDOPHOR SYSTEM OF THE- ATER TELEVISION," J. Soc.Motion Pier. and Telev. Engrs., Vol. 60, No. 4, pp. 337-343 (April1953).

2. E. Lindberg, SOLID CRYSTAL MODULATES LIGHT BEAM." Electronics (Dec.20, 1963).

3. L. B. Heilprm, COMMUNICATION ENGI- NEERING APPROACH TO MICROFORMS,American Documentation, Vol. 12, No. 3 (July 1961), p. 213.

As shown in FIG. 7, cathode ray tube 85 is similar to that shown in FIG.5 and is provided with internal grating 86 as heretofore described.Instead of a phosphor screen, however, the CRT is provided with a screen87 of material of controllable transparency of the type set forth above,for example, a scanned electro-optic ceramic plate. Intermediate screen87 and grating 86 is provided an evaporated aluminum film 88. Film 88 istransparent to the electron beam from the cathode ray tube 85 butreflects light. The aluminum film 88 can be formed along the innersurface of screen 87. A threedirnensional virtual image 9'0 can beobtained from the real time holography generated on screen 87 bycoherent light from laser 91. Laser light beam is incident on screen 87at an angle and the light passes through the screen material to aluminumfilm 88 from which it is reflected back through the screen material.According to the hologram transparency pattern generated on the screenthe coherent light is diffracted and interfered to reproduce athree-dimensional virtual image viewable from a station such as 92. Avariety of real time display systems can be constructed according to theabove principles. Thus, the film plane in FIG. I can be substituted witha screen of controlled transparency for imaging the hologram in realtime. The sequential holography reproduced on the screen can becontinuously interrogated with coherent light either reflectively ortransmissively for continuously reconstructing a threedimensional imageof the original object.

The various embodiments of the invention set forth in the foregoingexamples have suggested the use of CRTs for the amplitude patternimaging. Other electronically addressable optical displays can be used,however. As shown in FIG. 8, instead of a cathode ray tube. there isprovided a plate 100 of material comprising, for example, anelectro-optic modulator. As shown in this example, light from scannedlight source 101 is collimated by lens 102 and is normally incident onthe plate 100. The relative transparency of plate 100 to the light 102is controlled by the amplitude component of the electrical analog signalcorresponding to the image intensity function. This amplitude componentis derived from envelope detector 103 as heretofore described and isapplied to the modulator driver 104 which controls the transmission oflight through plate 100. The frequency of the amplitude component isrelatively low. The light plate 100 responds to the amplitude signalonly and does not have to resolve a high frequency spatial carrier orother high frequency components. The amplitude image generated by plate100 is processed in the manner heretofore described with reference, forexample, to FIG. 1. Instead of an electrooptic modulating plate, theplate can be, for example,

an electro-luminescent panel which generates its own scanned lightintensity in response to the applied signal.

What is claimed is: 1. A cathode ray tube of the type having an electronbeam generator, scanning deflection plates for controlling said electronbeam, and a screen of phosphor or controllable transparency responsiveto said electron beam, for reconstructing an image from holographiclight amplitude and phase information in electrical analog signal form,the improvement comprising:

phase deflection means and magnetic focusing means positioned along thepath of the electron beam generated by said cathode ray tube, andgrating means positioned within said cathode ray tube and close to butspaced from said controllable screen. 2. A cathode ray tube forreconstructing a hologram from holographic light amplitude and phaseinformation components in electrical analog signal form comprising:

electron beam generator means for generating an electron beam having anintensity responsive to the amplitude component of said electricalsignal;

scanning deflection means for scanning said electron beam in apredetermined raster;

a screen of controllable transparency responsive to said electron beam;

phase deflection means for controlling the angle of said electron beamin accordance with the phase component of said electrical signal; and

grating means positioned within said cathode ray tube and spaced frombut close to said controllable screen.

3. A cathode ray tube as set forth in claim 2 wherein said screen ofcontrollable transparency comprises an electro-optic ceramic plate.

4. A cathode ray tube as set forth in claim 3 wherein a layer ofmaterial transparent to the electron beam but reflective to light isinterposed between the grating means and the screen of controllabletransparency.

1. A cathode ray tube of the type having an electron beam generator,scanning deflection plates for controlling said electron beam, and ascreen of phosphor or controllable transparency responsive to saidelectron beam, for reconstructing an image from holographic lightamplitude and phase information in electrical analog signal form, theimprovement comprising: phase deflection means and magnetic focusingmeans positioned along the path of the electron beam generated by saidcathode ray tube, and grating means positioned within said cathode raytube and close to but spaced from said controllable screen.
 2. A cathoderay tube for reconstructing a hologram from holographic light amplitudeand phase information components in electrical analog signal formcomprising: electron beam generator means for generating an electronbeam having an intensity responsive to the amplitude component of saidelectrical signal; scanning deflection means for scanning said electronbeam in a predetermined raster; a screen of controllable transparencyresponsive to said electron beam; phase deflection means for controllingthe angle of said electron beam in accordance with the phase componentof said electrical signal; and grating means positioned within saidcathode ray tube and spaced from but close to said controllable screen.3. A cathode ray tube as set forth in claim 2 wherein said screen ofcontrollable transparency comprises an electro-optic ceramic plate.
 4. Acathode ray tube as set forth in claim 3 wherein a layer of materialtransparent to the electron beam but reflective to light is interposedbetween the grating means and the screen of controllable transparency.